The only known specimen of the new rodent: The 35 mm camera lens cap shows scale (Photo: Monica Romo)

By environment correspondent Alex Kirby

Zoologists say they have made a "dramatic" discovery in the Peruvian Andes - a hitherto unknown genus of mammal.

The discovery of the animal, a tree rat the size of a domestic cat, was made by Dr Louise Emmons, a researcher with the Smithsonian Institution in Washington DC.

She found it while climbing in the Vilcabamba mountains near the ruins of the Inca city of Macchu Picchu, an area which had not been researched before.

Dr Emmons was about 700 metres up the mountain when she came across the rat, which had just been killed by an Andean weasel.

Lucky strike

"I think I must have disturbed the weasel. It ran off and left the mammal almost literally at my feet.

"It was a really exciting discovery. I knew it was something I had never seen before, but it's wonderful to realise that this is a totally new genus of rodent."

Cuscomys is distinctively marked

She told BBC News Online: "It was tremendous luck for me that the weasel had just struck, because the rat is too big to be caught in the traps we set for mice and other small mammals, so I wouldn't have seen it but for the weasel.

"It was also extremely lucky the rat dropped exactly where it did. Another metre either way in that dense vegetation, and I'd have missed it."

Dr Emmons named the rat Cuscomys ashaninka, a reference both to the nearby city of Cusco and to the indigenous people of the area, the Ashaninka.

It is a powerfully-built animal, pale grey, with a white streak running along its head to its snout, and possesses large claws.

The expedition was organised by Conservation International (CI), based in Washington DC.

More to find

Dr LeeAnne Alonso of CI said: "This is an important discovery, and there are certain to be more of these animals out there."

"The fact than an animal so big remained undiscovered for so long makes you wonder what else remains undiscovered.

"It shows how elusive even quite large forest mammals can be. We didn't expect to find anything as dramatic as that."

Machu Picchu: Home of Cuscomys' relatives?

Louise Emmons says the tree rat's nearest relative is a large mammal which appears to have been kept as a pet by the Incas who lived at Machu Picchu.

Some have been found in tombs in the city's ruins, and it is thought they had been buried with their owners.

These Inca tomb rats were also large and adapted for living in trees. They are thought to be extinct, but Dr Emmons now believes some could still survive.

The Vilcabamba expedition notched up several other successes apart from finding Cuscomys.

It also found two new mouse species, two orchids, 11 butterflies and more than a dozen new species of frog and lizard.